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🛠️ Aligner Tool

Available from v2.1.0

This powerful tool allows you to align, rotate, scale, and skew vector features (lines or polygons) with precision. It works by mapping "source" vertices on your feature to corresponding "target" destination points on the map, supporting 1-point, 2-point, and 3-point transformations.

✅ Prerequisites​

  • An editable Line or Polygon layer must be the active layer in QGIS.
  • The layer must be in editing mode (pencil icon ✏️ activated).
  • For best results, enable snapping in Project > Snapping Options... to accurately select vertices and target coordinates.

🔄 Modes of Alignment​

Depending on the number of control point pairs you select, the tool performs different geometric operations:

1. One-Point Alignment (Translation / Move)​

  • What it does: Moves the entire feature to a new location without rotating or resizing it.
  • How to use:
    1. Pick one target point on the map.
    2. Pick the corresponding source vertex on your feature.
    3. Press Enter (or right-click) to complete the move.

2. Two-Point Alignment (Translate, Rotate & Optional Scale)​

  • What it does: Moves and rotates the feature to align with a baseline. You can choose whether to scale it.
  • How to use:
    1. Select the first target point and first source vertex.
    2. Select the second target point and second source vertex.
    3. A dialog box will appear asking you to choose:
      • Scale: Moves, rotates, and scales the feature so that the baseline matches the target distance exactly.
      • Align Only: Moves and rotates the feature while keeping its original size and shape.

3. Three-Point Alignment (Affine Transformation)​

  • What it does: Moves, rotates, scales, and skews/shears the feature. Ideal for fitting complex features into coordinates where skewing is necessary.
  • How to use:
    1. Select the first target point and first source vertex.
    2. Select the second target point and second source vertex.
    3. Select the third target point and third source vertex.
    4. The transformation will be calculated and applied automatically using a 3-point Affine algorithm.

🧭 Step-by-Step Instructions (Example: 2-Point Alignment)​

🟩 Step 1: Activate the Tool​

  1. Make sure your desired line or polygon layer is active and in editing mode.
  2. Click the Aligner Tool icon from the plugin toolbar or menu.
  3. The message bar will prompt:

    "Tool activated on layer <Layer Name>. Left-click to pick points, right-click to reset."

🟩 Step 2: Select the First Pair of Points​

  1. Select Target Point 1: Left-click on the map where you want the first vertex to end up. A red marker will appear.
  2. Select Source Vertex 1: Left-click on the vertex of the feature you want to move. A blue dashed line will connect the target point and source vertex.

🟩 Step 3: Select the Second Pair of Points​

  1. Select Target Point 2: Left-click on the map to define the second destination. A second red marker will appear.
  2. Select Source Vertex 2: Left-click on the second vertex on your feature. A second blue dashed line will appear.

🟩 Step 4: Perform the Transformation​

  • For 1-Point or 2-Point Alignment: If you have finished picking your control points (e.g., 2 pairs), press Enter (or right-click) to execute the alignment.
  • For 3-Point Alignment: Proceed to click the third target point and third source vertex. The alignment executes immediately upon the 6th click (3rd pair).

💡 Tips and Additional Features​

  • Multi-Feature Alignment: If you select multiple features in the active layer before activating the tool, the calculated transformation is applied to all of them together.
  • Resetting: A right-click at any time before the execution will cancel the current selection and reset the tool to step 0.
  • Visual Feedback:
    • Red Points: Show your target destination locations.
    • Blue Dashed Lines: Show the mapping from your source vertices to target points.
    • Magenta Cross: QGIS Snap indicator showing when you are successfully snapped to a vertex or edge.

🛑 To Exit the Tool​

Simply select any other tool from the QGIS toolbars (like the Pan or Select tool). The Aligner Tool will automatically deactivate.